Broadband Passive Sonar Signal Simulation in Shallow Ocean

The broadband plane wave model is valid only in the far-field of a point source under free-field propagating conditions. However the acoustics in ocean is characterized by multi-modal acoustic propagation due to its top-bottom limited boundary conditions. The effect of multi-modal field is to alter the source spectrum while the effect of dispersion is to modify the pulse shape. Moreover the use of a plane wave beamformer in a multi-modal field leads to a bias in the bearing estimates. These effects are highly dependant on the environment parameters and have important ramifications for target localization and classification in an ocean waveguide. We propose a more realistic simulator which essentially models these effects and therefore serves to provide test signals for first hand verification of signal processing algorithms to be developed for such scenarios. This model is to be understood as a better model than the naïve plane wave model which is entirely oblivious of even the gross features such as wave propagation in an oceanic waveguide. The channel parameter so estimated from the present simulation can be convolved with the radiated noise spectra of the source to generate the passive sonar signal.Defence Science Journal, 2011, 61(4), pp.370-376, DOI:http://dx.doi.org/10.14429/dsj.61.8

<span style="font-size:15.0pt;line-height:115%; font-family:"Times New Roman","serif";mso-fareast-font-family:Calibri; mso-ansi-language:EN-IN;mso-fareast-language:EN-US;mso-bidi-language:AR-SA">Methodologies for broadband reverberation data processing and analysis</span>

245-251<span style="font-size:9.0pt;line-height:115%; font-family:" times="" new="" roman","serif";mso-fareast-font-family:calibri;="" mso-ansi-language:en-in;mso-fareast-language:en-us;mso-bidi-language:ar-sa"="">Data processing methodologies used to analyse broadband reverberation signals from an explosive sound source are presented in this paper. Reverberation data was collected at deep (2070 m) and shallow water (57 m) sites in the Bay of Bengal. In these experiments, TNT charges (0.450 kg) were used as sound sources. The received signals were recorded using two hydrophones deployed from the ship. The sonic surface duct thickness at the shallow water site is around 32 m with a limiting ray angle of 3.16o and a lower cut off frequency at 416 Hz. The deep water location had deeper duct (63 m) with a limiting ray angle of about 4.47o and a lower cut off frequency of 177 Hz. One third octave band analysis using multirate filters and time frequency analysis were used to study the reverberation characteristics. Comparison of deep water and shallow water reverberation characteristic across a broad band of frequencies are presented along with analysis based on ray modelling.</span

Depth estimation of an underwater target by the method of time reversal mirror

231-236<span style="font-size:9.0pt;font-family: " times="" new="" roman","serif";mso-fareast-font-family:"times="" roman";mso-ansi-language:="" en-us;mso-fareast-language:ar-sa;mso-bidi-language:ar-sa"="" lang="EN-US">Time reversal mirror (TRM) refocuses the received signal back to the original source location regardless of the complexity of the medium of propagation. When the medium contains several reflectors, the time reversal process can be used to focus on the desired target. Two arbitrary scatterers are positioned in the oceanic waveguide at different ranges and depths. Assuming that range of the focused scatterer is known approximately, the converged transmission vector through TRM may be matched against the computed eigenvector corresponding to the largest eigenvalue of the modeled channel transfer matrix between the transmitter and a test scatterer (at test depth) in order to determine the depth of the focused scatterer. Note that this approach of matching with the focused transmission vector leads to an improved depth estimation performance due to better SNR gain over a conventional matched field processor without TRM.</span

Measured broadband reverberation characteristics in Deep Ocean

62-66Broad band reverberation measurements were collected in deep water (2067m) off Vizag. TNT scare charges (0.450 kg) were used as sound sources which were expended from the ship. The signals were recorded using two hydrophones deployed from the ship. The sound speed profile exhibits 63m duct with a limiting ray angle of about 4.47o and lower cut off frequency of 177 Hz. A flat plateau and then sudden fall of 10 dB near ~12-13s on the reverberation characteristic in deep water is related to the effect of sound speed profile and water depth. This strong reverberation return will have major consequences on the reverberation limited sonar performance. This effect is correlated with ray theory based model and the results are presented

Broadband Passive Sonar Signal Simulation in Shallow Ocean

<p class="MsoNormal" style="text-align: justify; mso-hyphenate: none;"><span style="mso-bidi-font-size: 10.0pt;">The broadband <em>plane wave model</em><span style="mso-bidi-font-style: italic;"> is valid only in the far-field of a point source under free-field propagating conditions</span>. However the acoustics in ocean is characterized by multi-modal acoustic propagation due to its top-bottom limited boundary conditions. The effect of multi-modal field is to alter the source spectrum while the effect of dispersion is to modify the pulse shape. Moreover the use of a plane wave beamformer in a multi-modal field leads to a bias in the bearing estimates. These effects are highly dependant on the environment parameters and have important ramifications for target localization and classification in an ocean waveguide. We propose a more realistic simulator which essentially models these effects and therefore serves to provide test signals for first hand verification of signal processing algorithms to be developed for such scenarios. This model is to be understood as a better model than the naïve plane wave model which is entirely oblivious of even the gross features such as wave propagation in an oceanic waveguide. The channel parameter so estimated from the present simulation can be convolved with the radiated noise spectra of the source to generate the passive sonar signal.</span></p><p class="MsoNormal" style="text-align: justify; mso-hyphenate: none;"><span style="mso-bidi-font-size: 10.0pt;"><strong>Defence Science Journal, 2011, 61(4), pp.370-376</strong><strong><strong>, DOI:http://dx.doi.org/10.14429/dsj.61.89</strong></strong><br /></span></p

A novel synthesis route for brookite rich titanium dioxide photocatalyst involving organic intermediate

© 2014, Springer Science+Business Media New York.High temperature stable brookite rich titanium dioxide of average crystallite size 20 nm has been prepared by a novel aqueous sol–gel method involving hydroxyethyl cellulose polymer (HEC) as an organic intermediate, wherein titania powder with brookite phase content as high as 44 wt% was obtained. The existence of brookite phase has been evident even after calcination of the samples at 900 °C, which also helped to maintain a specific surface area value of 5.5 m2g-1 compared to the surface area of 2.2 m2g-1 measured on pure titania sample with only rutile phase. The brookite rich titania exhibited superior photocatalytic activity under UV irradiation with a rate constant value of 0.011 min-1 compared to the value of 0.003 min-1 measured for pure rutile phase rich titania samples under similar conditions. The present study indicates that HEC assisted thermal decomposition can be an effective route to produce efficient photoactive brookite rich titania powders